The present invention relates to a frequency comparator circuit of the type incorporating means for the comparison of the signs A, F, F.sub.q of the respective instantaneous amplitudes of a signal received, of a reference signal of period T and a reference signal phase-displaced by T/4, whereby counting means supply a first signal proportional to the difference between the sum of the time slots where A differs from F and F.sub.q and where A is identical to F and F.sub.q and a second signal proportional to the difference between the sums of the time slots where A is different from F and identical to F.sub.q and where A is identical to F and different from F.sub.q.
The present invention is directed more particularly at selective calling devices used in radiotelephone communications or used for the selective calling of persons. In these devices, the telephone numbers are transmitted in the form of successive tones, each tone representing for example a digit. If a telephone whose number is composed of three digits, for example 3-9-2, receives a sequence of tones, to know whether the selective call is intended for it the telephone will investigate corresponding to the digit 3 then, if it finds it, the tone corresponding to digit 9, then, if the tone corresponding to the digit 9 immediately follows the tone corresponding to the digit 3, it will investigate the tone corresponding to the digit 2. Thus, the telephone must be able to recognise a predetermined tone, called the expected tone.
It is known to carry out this investigation of expected tones by means of a frequency comparator of the type indicated hereinbefore and in which the counting means perform a count from a given time. This given time is generally determined by a control pulse periodically supplied by a pulse generator belonging to the frequency comparator circuit.
Such frequency comparator circuits function satisfactorily, but do not make it possible to carry out with the said counting means counts which, at a given moment, relate to the duration of the expected tone. For this purpose, it will be necessary for the control pulse to be transmitted at the time of changing the tones, which is not generally possible, particularly when investigating a first tone of a sequence of expected successive tones and can only be due to chance. These frequency comparator circuits do not make it possible to fully take advantage of the fact that in general the duration of the tones received is known. They are consequently more sensitive to noise than in the case where the counts by the counting means could be systematically carried out for the duration of the tone.